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Member access operators - cppreference.com

Accesses a member of its operand.

Notes

• As with most user-defined overloads, return types should match return types provided by the built-in operators so that the user-defined operators can be used in the same manner as the built-ins. However, in a user-defined operator overload, any type can be used as return type (including void). One exception is operator->, which must return a pointer or another class with overloaded operator-> to be realistically usable.

Built-in subscript operator provides access to an object pointed-to by the pointer or array operand.

Built-in indirection operator provides access to an object or function pointed-to by the pointer operand.

Built-in address-of operator creates a pointer pointing to the object or function operand.

Member of object and pointer to member of object operators provide access to a data member or member function of the object operand.

Built-in member of pointer and pointer to member of pointer operators provide access to a data member or member function of the class pointed-to by the pointer operand.

The subscript operator expressions have the form

For the built-in operator, one of the expressions (either

or

) must be a glvalue of type “array of

” or a prvalue of type “pointer to

”, while the other expression (

or

, respectively) must be a prvalue of unscoped enumeration or integral type. The result of this expression has the type

.

2) The form with brace-enclosed list inside the square brackets is only used to call an overloaded operator[].

3) The form with comma-separated expression list inside the square brackets is only used to call an overloaded operator[].

The built-in subscript expression E1[E2] is exactly identical to the expression *(E1 + E2) except for its value category (see below) and evaluation order(since C++17): the pointer operand (which may be a result of array-to-pointer conversion, and which must point to an element of some array or one past the end) is adjusted to point to another element of the same array, following the rules of pointer arithmetic, and is then dereferenced.

When applied to an array, the subscript expression is an lvalue if the array is an lvalue, and an xvalue if it isn't(since C++11).

When applied to a pointer, the subscript expression is always an lvalue.

The type T is not allowed to be an incomplete type, even if the size or internal structure of T is never used, as in &x[0].

Using an unparenthesized comma expression as second (right) argument of a subscript operator is deprecated.

For example, a[b, c] is deprecated and a[(b, c)] is not.

An unparenthesized comma expression cannot be second (right) argument of a subscript operator. For example, a[b, c] is either ill-formed or equivalent to a.operator[](b, c).

Parentheses are needed to for using a comma expression as the subscript, e.g., a[(b, c)].

In overload resolution against user-defined operators, for every object type T (possibly cv-qualified), the following function signature participates in overload resolution:

#include <iostream>
#include <map>
#include <string>
 
int main()
{
    int a[4] = {1, 2, 3, 4};
    int* p = &a[2];
    std::cout << p[1] << p[-1] << 1[p] << (-1)[p] << '\n';
 
    std::map<std::pair<int, int>, std::string> m;
    m[{1, 2}] = "abc"; // uses the [{...}] version
}

Output:

The indirection operator expressions have the form

The operand of the built-in indirection operator must be pointer to object or a pointer to function, and the result is the lvalue referring to the object or function to which expr points. If expr does not actually points to an object or function, the behavior is undefined (except for the case specified by typeid).

A pointer to (possibly cv-qualified) void cannot be dereferenced. Pointers to other incomplete types can be dereferenced, but the resulting lvalue can only be used in contexts that allow an lvalue of incomplete type, e.g. when initializing a reference.

In overload resolution against user-defined operators, for every type T that is either object type (possibly cv-qualified) or function type (not const- or ref-qualified), the following function signature participates in overload resolution:

#include <iostream>
 
int f() { return 42; }
 
int main()
{
    int n = 1;
    int* pn = &n;
 
    int& r = *pn; // lvalue can be bound to a reference
    int m = *pn;  // indirection + lvalue-to-rvalue conversion
 
    int (*fp)() = &f;
    int (&fr)() = *fp; // function lvalue can be bound to a reference
 
    [](...){}(r, m, fr); // removes possible "unused variable" warnings
}

The address-of operator expressions have the form

If the operand is an lvalue expression of some object or function type

,

creates and returns a prvalue of type

, with the same cv qualification, that is pointing to the object or function designated by the operand. If the operand has incomplete type, the pointer can be formed, but if that incomplete type happens to be a class that defines its own

, it is unspecified whether the built-in or the overload is used. For the operands of type with user-defined

,

may be used to obtain the true pointer. Note that, unlike C99 and later C versions, there's no special case for the unary

applied to the result of the unary

.

If the operand is the name of an overloaded function, the address may be taken only if the overload can be resolved due to context. See

for details.

In overload resolution against user-defined operators, this operator does not introduce any additional function signatures: built-in address-of operator does not apply if there exists an overloaded operator& that is a viable function.

void f(int) {}
void f(double) {}
 
struct A { int i; };
struct B { void f(); };
 
int main()
{
    int n = 1;
    int* pn = &n;    // pointer
    int* pn2 = &*pn; // pn2 == pn
 
    int A::* mp = &A::i;      // pointer to data member
    void (B::*mpf)() = &B::f; // pointer to member function
 
    void (*pf)(int) = &f; // overload resolution due to initialization context
//  auto pf2 = &f; // error: ambiguous overloaded function type
    auto pf2 = static_cast<void (*)(int)>(&f); // overload resolution due to cast
}

The member access operator expressions have the form

The

must be an expression of

class type

.

2) The expr must be an expression of pointer to complete class type T*.

3,4) The expr must be an expression of scalar type (see below).

id-expr is a name of (formally, an identifier expression that names) a data member or member function of T or of an unambiguous and accessible base class B of T (e.g. E1.E2 or E1->E2), optionally qualified (e.g. E1.B::E2 or E1->B::E2), optionally using template disambiguator (e.g. E1.template E2 or E1->template E2).

If a user-defined operator-> is called, operator-> is called again on the resulting value, recursively, until an operator-> is reached that returns a plain pointer. After that, built-in semantics are applied to that pointer.

The expression E1->E2 is exactly equivalent to (*E1).E2 for built-in types; that is why the following rules address only E1.E2.

In the expression E1.E2:

If

is a

:

• If E2 is of reference type T& or T&&(since C++11), the result is an lvalue of type T designating the object or function to which the reference is bound.
• Otherwise, given the type of E2 as T, the result is an lvalue of type T designating that static data member.

Essentially, E1 is evaluated and discarded in both cases.

If

is a

:

• If E2 is of reference type T& or T&&(since C++11), the result is an lvalue of type T designating the object or function to which the corresponding reference member of E1 is bound.
• Otherwise, if E1 is an lvalue, the result is an lvalue designating that non-static data member of E1.
• Otherwise (if E1 is an rvalue(until C++17)xvalue (which may be materialized from prvalue)(since C++17)), the result is an rvalue(until C++11)xvalue(since C++11) designating that non-static data member of E1.

If

is not a

member, the

of the result is the union of the cv-qualifications of

and

, otherwise (if

is a mutable member), it is the union of the volatile-qualifications of

and

.

4) If E2 is a member enumerator, given the type of E2 as T, the result is an rvalue(until C++11)a prvalue(since C++11) of type T whose value is the value of the enumerator.

If

has a

and

is a

followed by the

or

designating the same type (minus cv-qualifications), optionally

, the result is a special kind of prvalue that can only be used as the left-hand operand of a function call operator, and for no other purpose

The resulting function call expression is called pseudo destructor call. It takes no arguments, returns void, evaluates E1, and ends the lifetime of its result object. This is the only case where the left-hand operand of operator. has non-class type. Allowing pseudo destructor call makes it possible to write code without having to know if a destructor exists for a given type.

operator. cannot be overloaded, and for operator->, in overload resolution against user-defined operators, the built-in operator does not introduce any additional function signatures: built-in operator-> does not apply if there exists an overloaded operator-> that is a viable function.

#include <cassert>
#include <iostream>
#include <memory>
 
struct P
{
    template<typename T>
    static T* ptr() { return new T; }
};
 
template<typename T>
struct A
{
    A(int n): n(n) {}
 
    int n;
    static int sn;
 
    int f() { return 10 + n; }
    static int sf() { return 4; }
 
    class B {};
    enum E {RED = 1, BLUE = 2};
 
    void g()
    {
        typedef int U;
 
        // keyword template needed for a dependent template member
        int* p = T().template ptr<U>();
        p->~U(); // U is int, calls int's pseudo destructor
        delete p;
    }
};
 
template<>
int A<P>::sn = 2;
 
struct UPtrWrapper
{
    std::unique_ptr<std::string> uPtr;
    std::unique_ptr<std::string>& operator->() { return uPtr; }
};
 
int main()
{
    A<P> a(1);
    std::cout << a.n << ' '
              << a.sn << ' '   // A::sn also works
              << a.f() << ' ' 
              << a.sf() << ' ' // A::sf() also works
//            << &a.f << ' '   // error: ill-formed if a.f is not the
                               // left-hand operand of operator()
//            << a.B << ' '    // error: nested type not allowed
              << a.RED << ' '; // enumerator
 
    UPtrWrapper uPtrWrap{std::make_unique<std::string>("wrapped")};
    assert(uPtrWrap->data() == uPtrWrap.operator->().operator->()->data());
}

Output:

If E2 is a non-static member and the result of E1 is an object whose type is not similar to the type of E1, the behavior is undefined:

struct A { int i; };
struct B { int j; };
struct D : A, B {};
 
void f()
{
    D d;
    static_cast<B&>(d).j;      // OK, object expression designates the B subobject of d
    reinterpret_cast<B&>(d).j; // undefined behavior
}

The member access operator expressions through pointers to members have the form

1) lhs must be an expression of class type T.

2) lhs must be an expression of type pointer to class type T*.

rhs must be an rvalue of type pointer to member (data or function) of T or pointer to member of an unambiguous and accessible base class B of T.

The expression E1->*E2 is exactly equivalent to (*E1).*E2 for built-in types; that is why the following rules address only E1.*E2.

In the expression E1.*E2:

if

is a pointer to data member,

• if E1 is an lvalue, the result is an lvalue designating that data member,
• otherwise (if E1 is an rvalue(until C++17)xvalue (which may be materialized from prvalue)(since C++17)), the result is an rvalue(until C++11)xvalue(since C++11) designating that data member;

2) if E2 is a pointer to member function, the result is a special kind of prvalue designating that member function that can only be used as the left-hand operand of a member function call operator, and for no other purpose;

3) cv-qualification rules are the same as for member of object operator, with one additional rule: a pointer to member that refers to a mutable member cannot be used to modify that member in a const object;

4) if E2 is a null pointer-to-member value, the behavior is undefined;

if the result

is an object such that its type is not

to the type of

, or its

does not contain the member to which

refers, the behavior is undefined;

6) if E1 is an rvalue and E2 points to a member function with ref-qualifier &, the program is ill-formed unless the member function has the cv-qualifier const but not volatile(since C++20);

7) if E1 is an lvalue and E2 points to a member function with ref-qualifier &&, the program is ill-formed.

In overload resolution against user-defined operators, for every combination of types D, B, R, where class type B is either the same class as D or an unambiguous and accessible base class of D, and R is either an object or function type, the following function signature participates in overload resolution:

R& operator->*(D*, R B::*);

where both operands may be cv-qualified, in which case the return type's cv-qualification is the union of the cv-qualification of the operands.

#include <iostream>
 
struct S
{
    S(int n) : mi(n) {}
    mutable int mi;
    int f(int n) { return mi + n; }
};
 
struct D : public S
{
    D(int n) : S(n) {}
};
 
int main()
{
    int S::* pmi = &S::mi;
    int (S::* pf)(int) = &S::f;
 
    const S s(7);
//  s.*pmi = 10; // error: cannot modify through mutable
    std::cout << s.*pmi << '\n';
 
    D d(7); // base pointers work with derived object
    D* pd = &d;
    std::cout << (d.*pf)(7) << ' '
              << (pd->*pf)(8) << '\n';
}

Output:

Subscript operator is overloaded by many standard container classes:

The indirection and member operators are overloaded by many iterators and smart pointer classes:

No standard library classes overload operator&. The best known example of overloaded operator& is the Microsoft COM class CComPtr, although it can also appear in EDSLs such as boost.spirit.

No standard library classes overload operator->*. It was suggested that it could be part of smart pointer interface, and in fact is used in that capacity by actors in boost.phoenix, but is more common in EDSLs such as cpp.react.

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

Operator precedence

Operator overloading

a = b
a += b
a -= b
a *= b
a /= b
a %= b
a &= b
a |= b
a ^= b
a <<= b
a >>= b

++a
--a
a++
a--

+a
-a
a + b
a - b
a * b
a / b
a % b
~a
a & b
a | b
a ^ b
a << b
a >> b

!a
a && b
a || b

a == b
a != b
a < b
a > b
a <= b
a >= b
a <=> b

a[...]
*a
&a
a->b
a.b
a->*b
a.*b

a(...)

a, b

a ? b : c

static_cast converts one type to another related type
dynamic_cast converts within inheritance hierarchies
const_cast adds or removes cv-qualifiers
reinterpret_cast converts type to unrelated type
C-style cast converts one type to another by a mix of static_cast, const_cast, and reinterpret_cast
new creates objects with dynamic storage duration
delete destructs objects previously created by the new expression and releases obtained memory area
sizeof queries the size of a type
sizeof... queries the size of a pack (since C++11)
typeid queries the type information of a type
noexcept checks if an expression can throw an exception (since C++11)
alignof queries alignment requirements of a type (since C++11)

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